These studies concern the interaction of calcium with plasma membranes. Through a lucky accident we have discovered a technique for localizing calcium ions in association with membranes at the ultrastructural level. The technique is simple and thus can be utilized by many investigators for the study of diverse problems. Current work involves refining the technique, determining the physiological significance of the deposits we observe adjacent to membranes, and applying the method to systems in which calcium ions play a key role in regulation of cell function.